Doug K.
Bill Flemings book on British Sporting Rifle Cartridges has no less than three pages devoted to this round. There were many variants in length and construction, the earliest being coiled brass foil like the early .577 Snider and .577/450 Martini-Henry service rounds.
I would advise you to remove the Forearm and take the barrels off. On the underside of the breech end there will be proof Marks and also on the Actions Water Table. These will indicate whether it is Black Powder or Nitro proved. Often the bullet weight and the Powder charge for which the rifle is regulated is included in the proof markings. It's quite a potent cartridge and fully capable of taking any North American game and predators up to and including Grizzly and Polar bears. Not so sure how good it will be on Sasquatch in its BP loading. Lol. If the rifle is BP proof don't use any other powder. It isn't worth the risk, and clean up is easy.
Harry
